1. Field of the Invention
The present invention relates to an improvement of an alarm device for internal pressure of a tire, the alarm device monitoring a state of air pressure of each tire mounted on a body of a vehicle and notifying abnormalities of the internal pressure to a driver when the main switch of the vehicle is off or the vehicle stops.
2. Description of the Related Art
Traditionally, such an alarm device has been proposed. In the alarm device, a transmitting-side module is provided inside of the tire, the transmitting-side module comprising; a sensor unit having at least a pressure sensor for detecting the internal pressure of the tire and tire rotational-movement detecting means for detecting a rotational movement of said tire (such as a centrifugal force sensor); a signal-processing unit for processing signals detected at the pressure sensor and the tire rotational-movement detecting means; a transmitting unit for transmitting a tire internal-pressure information signal with a first duration through a transmitting antenna, the tire internal-pressure information signal having a tire internal-pressure data generated in and output from the signal-processing unit based on a detection signal from the pressure sensor; and a power supply for supplying an electric power to the sensor unit, the signal-processing unit and the transmitting unit; and a receiving-side module is provided at a vehicle side, the receiving-side module comprising; a receiving unit for receiving the tire internal-pressure information signal wireless transmitted from the transmitting antenna of the transmitting unit; and a signal-processing unit for processing tire internal-pressure data supplied from the receiving unit and outputting an alarm signal (see page 1 and FIG. 1 of JP-A-2001-80321, for example).
FIG. 1 is a diagrammatic cross-sectional view showing a state where the transmitting-side module of such an alarm device is mounted on a tire rim. This example of a transmitting-side module 1 is attached to a well section side 5 of a rim 4, integrally with a cylindrical valve device 3 for applying internal pressure to a tire 2. In this way, the transmitting-side module 1 is arranged as close as possible to an outer peripheral surface of a well section bottom 6 within the well section of the rim, so as not to interfere with the tire 2 when mounting the tire 2 onto the rim 4.
A transmitting antenna built-in the transmitting-side module 1 transmits the tire internal-pressure information signal. A receiving antenna is provided at a body side of a vehicle and receives the tire internal-pressure information signal. The tire internal-pressure information signal received by the receiving antenna is processed by the signal-processing unit. When abnormalities of the internal pressure of the tire exist, an alarm signal is generated. The abnormalities of the internal pressure of the tire are informed to the driver by the alarm signal (for example, by turning on an alarm lamp).
While the sensor unit of the transmitting-side module 1 arranged inside the rim 4 is provided with the pressure sensor and centrifugal force sensor as described above, in some cases, the sensor unit is also provided with: a temperature sensor for confirming that the operating temperatures of the above sensors are within predetermined ranges, respectively; and/or a voltage sensor for confirming that the voltage of the electric-power source is not less than a predetermined level. FIG. 1 shows a pressure sensor (the reference numeral thereof is not shown) among such sensors. These sensors typically consist of semiconductor sensors requiring electric power for operation. Further, the electric power is also consumed in the signal-processing unit and transmitting unit, for example. As a result, a large amount of electric power of the electric-power source is consumed and thus a life of the electric-power source is reduced if the electric power is always supplied to these sensor unit, signal-processing unit and transmitting unit. It is necessary to consume the electric power as small as possible because the electric-power source of the transmitting-side module 1 consists of a battery (not shown) and it is not easy to exchange the old battery with new one.
It has been proposed to drive the transmitting-side module intermittently in order to reduce the electric power consumption therein. Normally, since the change of the internal pressure of the tire does not occur rapidly, it is not necessary to transmit the tire internal-pressure information signal frequently and thus it is possible to reduce the electric power consumption by transmitting the tire internal-pressure information signal from the transmitting-side module to the receiving-side module at intervals of 60 seconds, for example.
On the other hand, the receiving-side module provided at the body side of the vehicle is powered from a battery mounted on the vehicle. Therefore, the disadvantage of the electric power consumption is not so severe as the transmitting-side module. However, there is a tendency to increase the number of electrical equipments and thus it is not preferable to keep, the receiving-side module on when the vehicle stops. As a result, it has been proposed to keep the receiving-side module off entirely when the vehicle stops. However, for example, when the tire has picked up a nail, the internal pressure reduces gradually and thus the internal pressure does not reduce to a certain value during the moving of the vehicle but reduces to a certain value at the long time stop of the vehicle.
In this case, if it is impossible to monitor the internal pressure when the receiving side module keeps off, the vehicle may restart with a low internal pressure of the tire. It is necessary to avoid the movement of the vehicle with the low internal-pressure of the tire as much as possible because the tire may be damaged frequently and considerably even if the movement of the vehicle is very short. Of course, although the operation of an apparatus for monitoring the internal pressure of the tire is started by turning on the main switch at the start of the vehicle, the vehicle may start before starting of the operation of the apparatus because the considerable reduction of the internal pressure of the tire is not represented immediately. Therefore, it is desirable to detect the internal pressure of the tire by the transmitting-side module even when the vehicle stops, transmit the information obtained from the detection of the internal pressure of the tire to the receiving-side module and to monitor the internal pressure of the tire constantly.
In this way, it is desirable to reduce the electric power consumption by driving the transmitting-side module and receiving-side module intermittently if the monitoring of the internal-pressure of the tire is performed even when the vehicle stops. However, it is impossible to synchronize the transmitting-side module and receiving-side module with each other and thus there is a disadvantage that the tire internal pressure information signal transmitted from the transmitting side module may be received by the receiving side module with a reliability. In order to improve the probability of receipt of the tire internal pressure data included in the tire internal-pressure information signal, it is conceivable to reduce the intervals of intermittent operations of the receiving side module. However, it is impossible to reduce the electric power consumption of the receiving side module effectively by the reduction of the intervals of intermittent operations of the receiving side module.